The present invention is generally directed to methods for removing a coating on a substrate. More particularly, the invention relates to the removal of overlay or diffusion coatings on a metal substrate, e.g., a superalloy component.
As operating temperatures of gas turbine engines increase to achieve improved fuel efficiency, advanced oxidation-resistant coatings are required for better environmental protection, as well as improved thermal barrier coating life. Current coatings used on components in gas turbine hot sections, such as blades, nozzles, combustors, and transition pieces, generally belong to one of two classes: diffusion coatings or overlay coatings.
State-of-the-art diffusion coatings are generally formed of aluminide-type alloys, such as nickel-aluminide, platinum-aluminide, or nickel-platinum-aluminide. Overlay coatings typically have the composition MCrAl(X), where M is an element from the group consisting of Ni, Co, Fe, and combinations thereof, and X is an element from the group consisting of Y, Ta, Si, Hf, Ti, Zr, B, C, and combinations thereof. Diffusion coatings are formed by depositing constituent components of the coating, and reacting those components with elements from the underlying substrate, to form the coating by high temperature diffusion. In contrast, overlay coatings are generally deposited intact, without reaction with the underlying substrate.
It has become commonplace to repair turbine engine components, particularly airfoils, and return those components to service. During repair, any coatings are removed to allow inspection and repair of the underlying substrate. Removal is typically carried out by immersing the component in a stripping solution containing an acid, such as a mixture of strong mineral acids (e.g., hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid), as well as other additives.
However, some of the stripping compositions of the prior art do not remove sufficient amounts of the coatings. Further time and effort is thus required to complete the removal (e.g., by grit blasting), and this can in turn lead to a decrease in the efficiency of the repair process. Furthermore, some of the compositions that do sufficiently remove the coatings also attack the base metal of the substrate, pitting the base metal, or damaging the metal via intergranular boundary attack. Moreover, conventional stripping solutions often emit an excessive amount of hazardous, acidic fumes. Due to environmental, health and safety concerns, such fumes must be scrubbed from ventilation exhaust systems.
It is thus apparent that new processes for removing coatings from substrates (e.g., metal substrates) would be welcome in the art. The processes should be capable of removing substantially all of the coating material, while not attacking the substrate itself. It would also be desirable if the processes did not result in the formation of an unacceptable amount of hazardous fumes. Moreover, the processes should be capable of removing a substantial amount of coating material that might be located in indentations, hollow regions, or holes in the substrate, e.g., passage holes in a superalloy substrate.
One embodiment of the invention is directed to a method for selectively removing at least one coating from the surface of a substrate, comprising the step of contacting the coating with an aqueous composition which comprises an acid having the formula HxAF6, or precursors to said acid. Usually, A is selected from the group consisting of Si, Ge, Ti, Zr, Al, and Ga; and x is 1-6. The acid is typically present at a level in the range of about 0.05 M to about 5 M. In some preferred embodiments, the aqueous composition comprises the compound H2SiF6 or H2ZrF6. As described below, these compounds may sometimes be formed in situ.
In some embodiments, the aqueous composition further comprises at least one additional acid or precursor thereof. The additional acid usually has a pH of less than about 7 in pure water, and preferably, less than about 3.5. A variety of these secondary acids can be used, and phosphoric acid is often preferred.
In preferred embodiments, the substrate is immersed in a bath of the aqueous composition, under temperature and time conditions sufficient to selectively remove the coating. As used herein, xe2x80x9cselective removalxe2x80x9d of the coating (or coatings) refers to the removal of a relatively large percentage of the coating, while removing only a very small portion (or none) of the substrate material, and while not adversely affecting the substrate in any substantial manner.
The coating being removed from the substrate usually comprises at least one diffusion coating or overlay coating, e.g., an aluminide-type coating or an MCrAl(X) material, respectively. Moreover, the substrate is usually a metallic material or a polymeric material, and is often in the form of a superalloy component.
Another embodiment of the invention is directed to an aqueous composition for selectively removing a coating from the surface of a substrate, comprising an acid having the formula HxAF6, or precursors for said acid, wherein A is selected from the group consisting of Si, Ge, Ti, Zr, Al, and Ga; and x is 1-6. The acid is usually present in the composition at levels described hereinafter. As mentioned above and further described below, at least one additional acid may be used in conjunction with the primary acid.
Further details regarding the various features of this invention are found in the remainder of the specification.